Large Scale Shifts in Ecosystems

A growing body of empirical evidence suggests that ecosystems may shift abruptly among alternative states.

In fact, large-scale shifts in ecosystems have been observed in lakes, coral reefs, woodlands, desserts, and oceans. For example, a distinct shift occurred in the Pacific Ocean ecosystem around 1977 and 1989. Abrupt changes in the time series of fish catches, zooplankton abundance, oyster condition, and other marine ecosystem properties signified conspicuous shifts from one relatively stable condition to another (Figure 6). Also termed 'regime shifts', the implications of these abrupt transitions for fisheries and oceanic CO2 uptake are profound, yet the mechanisms driving these shifts remain poorly understood. It appears that changes in oceanic circulation driven by weather patterns can be evoked as the dominant causes of this state shift. However, competition and predation are becoming increasingly recognized as important drivers of change altering oceanic community dynamics. In fact, fisheries are well known to affect entire food webs and the trophic organization of ecosystems. Therefore, one could imagine that the sensitivity of a single keystone species to subtle environmental change could cause major shifts in community composition. Given this interplay between and within the biotic and abiotic components of an ecosystem, resolving the causes of regime shifts in oceanic ecosystems will likely require an understanding of the interactions between the effects of fisheries and the effects of physical climate change.

1977 Regime shift 1.0

1977 Regime shift 1.0

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1989 Regime shift 1.0

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Figure 6 Distinct shifts in ecosystem states, also referred to as 'regime shifts', occurred in the Pacific Ocean ecosystem around 1977 and 1989. The ecosystem state index shown here was calculated based on the average of climatic and biological time series. From Scheffer M, Carpenter S, Foley JA, Folke C, and Walker B (2001) Catastrophic shifts in ecosystems. Nature 413: 591-596.

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Figure 6 Distinct shifts in ecosystem states, also referred to as 'regime shifts', occurred in the Pacific Ocean ecosystem around 1977 and 1989. The ecosystem state index shown here was calculated based on the average of climatic and biological time series. From Scheffer M, Carpenter S, Foley JA, Folke C, and Walker B (2001) Catastrophic shifts in ecosystems. Nature 413: 591-596.

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